The invention relates to a printing attachment for rotary printing presses, in particular for indirect letterpress printing, such as dry offset or the like, having cylinders interposed between parts of the frame, a first and a second cooperating cylinder having associated with them at each of their ends a support member, e.g. an annular or disc-shaped support member, held independently of the rotation of the cylinders, and outer support surfaces on two support members which in each case face one another.
In the case of a known printing attachment (German Gebrauchsmuster [Utility Model] No. 79 18 882) the lateral journals of the forming cylinder and of the rubber cylinder of an offset printing press are mounted by rolling bearings in lateral frames. Bearing rings are secured on the journals outside the latter, while support rings, which do not participate in the rotation of the cylinders, are mounted on the journals inside the walls. In the case of one of the two cylinders the main bearings are arranged in bushes with an eccentric outer surface, which may be rotated in corresponding receiving bores in the lateral frame and may be fixed in a position in which a desired prestressing force is produced in each case. The two cylinders are thus pressed towards one another in conventional manner. The purpose of such a design of the printing attachment should be to provide a support for printing attachment cylinders which performs an operation corresponding to the bearing ring operation, but provides better repair conditions.
In the past it was generally assumed in the printing industry that in principle all cylinders of a printing attachment must be pressed against one another with a specific force. This likewise applies to different printing methods. As stated for example in the paper "Die `Kiss-print-Methode` im Offsetdruck" [The kiss-print method in offset printing], in the journal "Der Polygraph" [The polygraph], 23-80, p. 2161, a "proper pressure" or "compressive stress" is considered necessary for the process of ink transfer in the printing press, namely between all the cylinders of the printing attachment. Here the established opinion is expressly stated that the processes of ink acceptance and ink transfer must take place at a specific pressure or can only be correctly carried out under pressure. A slight contact of the inked printing plate and the rubber blanket would not be sufficient. The so-called kiss print impression, in which a trouble-free pressing out of the surface is made possible, should be between an excessive and an insufficient impression. This would be a proper, adequate contact between the cylinders and not merely a rapid, superficial contact. Calibrated base sheets, coatings etc. are mentioned as auxiliary means for attaining such a sufficient pressure between the cooperating cylinders. This means, however, that the pressure which should be necessary, according to current practice to form a good printed image must be produced in the case of difficult work.
In the case of the invention it has been recognized that current practice is not suitable for solving the problem of the correct ink transfer and trouble-free printing. The pressures generally thought to be necessary and applied in the case of cooperating cylinders lead to so-called wearing bands, i.e. flat pressure surfaces on the cylinders, with a width of from 0.6 to 0.7 mm for example in the case of a cylinder diameter of approximately 200 mm. It has been found that in the case of cylinders which are applied to one another with such pressure or a similar pressure there is a squeezing away of the ink from the "point", so that at least to a large extent the ink does not stay where it should in fact be, but is pushed aside into the periphery of this point. This explains, in the case of indirect letterpress printing for example, a reduced brilliance of the ink, a dull or pale image or the like. According to the teaching of the invention it is therefore correct for the ink, as it moves from one cylinder to the other during the transfer, to be kept or concentrated only on the "spot", i.e. at the position which actually belongs to the regular printed image in the same way as innumerable other ones and is to contribute to the printed result. Only in the case of a printing method which can carry this out is it possible correctly to speak of a kiss-print method.
By means of the invention it should be possible to solve as far as possible the problems discernable from the above.